1. Field of the Invention
The present invention relates generally to a torque sensor for monitoring a rotary torque applied to a rotating body. More specifically, the invention relates to a magnetostriction type torque sensor for detecting magnetostriction caused by application of rotary torque on the rotating body, such as a drive shaft of an automotive vehicle, a steering column shaft or a steering shaft in an automotive steering system, an input clutch shaft or other rotary shaft in an automotive power transmission, a spindle of a machine tool, a rotary shaft of an electric motor and so forth. Further particularly, the invention relates to compensating for errors caused by temperature variation in the measurement of applied torque.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 61-127952 and Japanese Patent First Publication (Tokkai) Showa No. 61-17953 disclose previously proposed torque sensors of the type which are for front contact with the rotating body in which the torque is being measured. In these prior art arrangements, one or more sensor elements are disposed about the rotating body in an automotive power transmission casing and secured to ribs, flanges or the like of the casing by way of screws or the like.
These sensor arrangements are arranged to project from their mounting sites and juxtapose the peripheral outer surface of the shaft in which torque is to be sensed and to include U-shaped cores on which exciting and measuring coils are wound. Theses torque sensors have suffered from the drawback that as they are exposed to only a portion of the rotating shaft, they tend to be subject to "eccentricity" noise which is generated when the shaft is subject to the application of torque which induces twisting deformation and causes the portion of the shaft to which the sensor is exposed to become momentarily eccentric.
On the other hand, Japanese Patent First Publication (Tokkai) Showa No. 62-185136 discloses the monitoring of a rotary torque applied to an object shaft, in which is provided axially spaced sections of magnetic anisotropy by providing mutually different directions of magnetic fields. Opposing these sections, torque detector coils are provided. The torque detector coils are so designed as to monitor magnetostriction magnitude caused by twisting deformation of the associated sections of the object shaft which is caused by application of the rotary torque. The torque detector coils are further designed to generate output signals corresponding to the detected magnetostriction magnitude.
The torque sensor shown tends to be subject to the influence of heat causing error in the measurement of the applied torque. Namely, in case the torque sensor is used for monitoring driving torque at the engine output shaft or rotary shaft in a power transmission, heat created by combustion in the engine is apt to be transmitted. Similarly, in the case of a machining tool, heat created in an electric motor tends to be transmitted to the rotary shaft. In the alternative, when the rotary shaft is supported by means of a bearing, friction between the outer periphery of the rotary shaft and the bearing tends to create heat. Such heat transmitted to the rotary shaft causes a temperature gradient in the axial direction of the rotary shaft. The temperature difference in different portions of the shaft causes a difference in magnetic permeability. This results in variation of impedance in respective detector coils to cause an error in the output signals of the coils. Such a temperature dependent error clearly degrades accuracy in measurement of the applied torque.